There are several applications which require the accurate and remote placement of small quantities of liquid metal. One of these is in the application of solder “bumps” to the terminals of integrated circuits where pure tin is of interest because it is more environmentally friendly than lower temperature solder alloys which contain lead, cadmium or other toxic metals. See, for example, U.S. Pat. No. 5,229,016, issued Jul. 20, 1993 to Hayes et al., and U.S. Pat. No. 5,415,679, issued May 16, 1995 to Wallace et al. Another requirement is in the generation of extreme ultraviolet (EUV) radiation for use in lithography where a small droplet of tin has to be positioned at the focus of a laser beam, or in the center of a small pinch discharge, as disclosed in U.S. application Ser. No. 10/911,334, filed Aug. 4, 2004 and entitled “Injection Pinch Discharge Extreme Ultraviolet Source”. Tin is chosen because its ionized states emit strongly at 13.5 nm, the wavelength of choice for EUV lithography. Both of these applications require tin droplets in the range of 10 μm-100 μm in diameter. The droplets should have relatively high velocity, so that high repetition rate processes can be performed. The positional accuracy typically has to be better than 10% of a droplet diameter at several centimeters from the point of launch. Such a degree of precision is necessary for laser-produced plasma EUV sources, in which the focused laser beam must be intercepted by a tin droplet. A lesser degree of precision is required for the injection pinch EUV source, in which the tin has to be positioned within a pinch discharge that may be much larger than the tin droplet.
Prior art methods of liquid drop generation often use the contraction of a piezoelectric or electrostrictive tube in response to an applied electric field. The liquid is contained within the tube and there is a small orifice at one end. The electric field causes contraction of the tube material and forces a small quantity of liquid through the orifice. In the case of liquid metals the operating temperature is often above the Curie temperature at which the piezoelectric effect loses its strength. The use of an electrostrictive transducer has been proposed in order to allow long duration operation at elevated temperature. However, there appear to be limits to the velocity with which the droplets may be ejected, and to the ultimate temperature range accessible by these techniques.
In an approach that avoids the use of temperature-dependent transducer materials a pressure impulse is applied to liquid inside a nozzle via a barrier membrane, the outer side of which is pressurized by a pulsed electromagnetic field, as disclosed in U.S. Pat. No. 4,057,807, issued Nov. 8, 1977 to Fishbeck et al. The pressure of the field moves the membrane which in turn pushes the liquid through the nozzle generating droplets. Because of the mass of the membrane and its size, the pressure available via this method is not as great as would be desired, especially for the generation of the very small, high velocity droplets required in an EUV source.
Accordingly, a new principle of liquid metal droplet generation is needed that circumvents one or more of the above difficulties.